A photodiode may provide a spectral response for photon to current conversion that does not match a desired response for the application of the photodiode. For example, a complementary metal-oxide-semiconductor (CMOS) photodiode produces a spectral response that does not match the response of a human eye (called photopic response). In this example, the greatest mismatch of the spectral response of a CMOS photodiode and a photopic response occurs at the near infrared (NIR) spectrum, where the CMOS photodiode has a relatively high sensitivity compared to a human eye.
Different techniques have been used to tune photodiodes to produce the desired spectral responses. As one example, a tuned optical filter may be positioned between the light source and the photodiode. As another example, Fabry-Pérot interferometers or etalons may be used to tune photodiodes to produce the desired spectral responses. As a further example, a sensor may include multiple photodiodes in a stacked configuration, each photodiode in the stack providing a different spectral response. The currents from each photodiode in the stack may be compared to determine the intensities of the light source across the spectrum in order to facilitate compensation of the electrical responses of the stacked photodiodes in order to produce a desired spectral response.